PURPOSE: To attain the detection of interference stably under prescribed conditions independently of a reception level of a desired wave, by detecting a reception signal level before angular demodulation and varying a discrimination level in response to this detection level.

CONSTITUTION: When a reception signal level of the desired wave is small as shown in a figure I , the level is detected at a rectifier circuit 22 of a discrimination level control section 2 and a large discrimination voltage VH is generated from a DC inversion circuit 23 in response to the detection level. When the reception signal level is large as shown in a figure II, a small discrimination voltage as shown in a figure VL is generated. Since the discrimination voltage of a comparator 3 is controlled small when the reception signal level of the desired wave is large and controlled large when small, even if the level of the desired wave is changed, a D/U detection point is kept constant at all times, allowing to attain stable detection of interference.